Discrete Math Seminars

Most Tuesdays in K9509 - 12:30pm to 1:20pm

Please join us for this weekly seminar on a wide variety of topics under the umbrella of discrete mathematics. We gratefully acknowledge the Pacific Institute of Mathematical Sciences for their generous funding since 2010.

Watch for upcoming Seminars listed below, offered in SCK 9509 and/or on Zoom. We welcome remote participation from groups at other universities.

All talks will be announced on this page, on the department calendar (found at the bottom of the Math homepage), and on the DM Seminar mailing list. If you have an SFU email address, you can subscribe or unsubscribe to the mailing list by logging in to maillist.sfu.ca and searching for the math-dmsem list. Otherwise, you can ask any discrete math faculty member to manually add you to the list. With the permission of the speaker, we will record the talk video and upload it to the SFU Discrete Math Seminar Youtube Channel.

Current organizer: Ladislav Stacho & Alexander Clow

2024 Talks

Spring 2024

Date: Tuesday April 09, 2024	Speaker: Kasra Masoudi, SFU	Talk Title: A Strict Inequality on the Energy of Edge Partitioning of Graphs
There is an interesting connection between chemistry and graph theory. This connection is highlighted by the relationship between a energy of a molecule and the eigenvalues of its graph. In the 1970s, a chemist named Ivan Gutman introduced a concept to define the energy of graphs, denoted as E(G). Simply put, Gutman’s definition involves adding up the absolute values of the eigenvalues of the graph’s adjacency matrix A. There exists a famous inequality on the energy of edge partitioning of Graphs, which states that energy of a graphs G is either less than or equal to the sum of the energies of its edge partitions. In this work, we show that in case of the connectivity of G the inequality is strict by using some features of positive semi-definite matrices and the relationship of the partitions with their adjacency matrices.
Date: Tuesday April 02 , 2024	Speaker: CANCELLED	Talk Title:

Date: Tuesday March 26 , 2024	Speaker: Vesna Irsic, University of Ljubljana	Talk Title: Domination of subcubic planar graphs with large girth
Since Reed conjectured in 1996 that the domination number of a connected cubic graph of order n is at most \lceil \frac13 n \rceil, the domination number of cubic graphs has been extensively studied. It is now known that the conjecture is false in general, but Henning and Dorbec very recently showed that it holds for graphs with girth at least 9. Zhu and Wu stated an analogous conjecture for connected cubic planar graphs. In this talk we present a new upper bound for the domination number of subcubic planar graphs: if G is a subcubic planar graph with girth at least 8, then \gamma(G) < n_0 + \frac{3}{4} n_1 + \frac{11}{20} n_2 + \frac{7}{20} n_3, where n_i denotes the number of vertices in G of degree i, for i \in \{0,1,2,3\}. We also discuss possible improvements of this bound and highlight the main ideas of the proof. Joint work with Eun-Kyung Cho, Eric Culver and Stephen G. Hartke.
Date: Tuesday March 05 , 2024	Speaker: Amarpreet Rattan, SFU	Talk Title: Symmetry in star factorisations (continued)
On Feb 14, J. Campion Loth gave the Discrete Math seminar on "Symmetry of Star Factorisations", where he focused on the combinatorial aspects of these objects and related factorisation problems. In this talk, we will look at these objects, which I will quickly reintroduce, from the point of view of algebra and the space of symmetric functions. I will give a brief but gentle introduction to symmetric functions, Jucys-Murphy elements, and their relationship to star and monotone factorisations. This is joint work with J. Campion Loth.
Date: Tuesday February 20, 2024 CANCELLED	Speaker: Peter Horak, University of Washington	Talk Title: Perfect Matchings in Cubic Graphs
In 2008 Alon and Friedland showed that a simple cubic graph G on 2n vertices has at most 6^{n/3} perfect matchings, and this bound is attained by taking the disjoint union of bipartite complete graphs K_{3,3}. In other words, the above theorem says that the complete bipartite graph K_{3,3} has the highest density of perfect matchings among all cubic graphs; thus the disjoint union of its copies constitutes the extremal graph. However, this result does not provide any insight into the structure of extremal connected cubic graphs. In this talk it will be presented that for n >= 6, the number of perfect matchings in a simple connected cubic graph on 2n vertices is at most 4f_{n-1}, with f_{n} being the n-th Fibonacci number, and a unique extremal graph will be characterized as well. In addition, it will be shown that the number of perfect matchings in any cubic graph G equals the expected value of a random variable defined on all 2-colorings of edges of G. In the second part of our talk an improved lower bound on the maximum number of cycles in a cubic graph will be provided.
Tuesday, February 13 , 2024	Sepeaker: Jesse Campion Loth, University of Bristol	Talk Title: Symmetry of star factorisations
How many ways are there to write a given permutation as a product of m transpositions (i_1, j_1) (i_2, j_2) \dots (i_m, j_m)? For example, there are three ways to write (1,2,3) as a product of two transpositions: (1,2)(1,3), (1,3)(2,3) and (2,3)(1,2). This is an example of a permutation factorisation problem. There are a broad range of problems of this type, with links to algebraic geometry and graph embeddings. We will first give an overview of how problems of this type are usually approached using generating functions and character theory. We then show how a combinatorial approach can be used to give bijections between different classes of permutation factorisation. In particular, we will present the first fully combinatorial proof of the symmetry of star factorisations, answering a question of Goulden and Jackson [2009]. This is joint work with Amarpreet Rattan, who will give a seminar on a symmetric function approach to these problems in March.
Date: Tuesday Feb 06 , 2024	Speaker: Torsten Mutze, University of Warwick	Talk Title: Kneser graphs are Hamiltonian
For integers k>=1 and n>=2k+1, the Kneser graph K(n,k) has as vertices all k-element subsets of an n-element ground set, and an edge between any two disjoint sets. It has been conjectured since the 1970s that all Kneser graphs admit a Hamilton cycle, with one notable exception, namely the Petersen graph K(5,2). This problem received considerable attention in the literature, including a recent solution for the sparsest case n=2k+1. The main contribution of our work is to prove the conjecture in full generality. We also extend this Hamiltonicity result to all connected generalized Johnson graphs (except the Petersen graph). The generalized Johnson graph J(n,k,s) has as vertices all k-element subsets of an n-element ground set, and an edge between any two sets whose intersection has size exactly s. Clearly, we have K(n,k)=J(n,k,0), i.e., generalized Johnson graphs include Kneser graphs as a special case. Our results imply that all known families of vertex-transitive graphs defined by intersecting set systems have a Hamilton cycle, which settles an interesting special case of Lovász’ conjecture on Hamilton cycles in vertex-transitive graphs from 1970. Our main technical innovation is to study cycles in Kneser graphs by a kinetic system of multiple gliders that move at different speeds and that interact over time, reminiscent of the gliders in Conway’s Game of Life, and to analyze this system combinatorially and via linear algebra. This is joint work with my students Arturo Merino (TU Berlin) and Namrata (Warwick).

Fall 2023

Date: Tuesday December 5, 2023	Speaker: Omer Angel, UBC	Talk Title: Non-monotonicity for all-in poker tournaments
We consider a simplified model for a poker tournament taking into account only hands where two or more players make the maximal bet of their entire stack. While the probability of any player being the overall winner is a simple function of the amounts they currently have, the probability of ending up in kth place for any other k is a surprisingly complex function, which we study. Joint work with Mark Holmes.
Date: Tuesday November 28, 2023	Speaker: Yanwen Luo, SFU	Talk Title: Spaces of Geodesic Triangulations of Surfaces
In 1962, Tutte proposed a simple method to produce a straight-line embedding of a planar graph in the plane, known as Tutte's spring theorem. This construction provides not only one embedding of a planar graph, but infinite many distinct embeddings of the given graph. This observation leads to a surprisingly simple proof of a classical theorem proved by Bloch, Connelly, and Henderson in 1984 stating that the space of geodesic triangulations of a convex polygon is contractible. In this talk, we will introduce spaces of geodesic triangulations of surfaces, review Tutte's spring theorem, and present this short proof. We will briefly report the recent progress in identifying the homotopy types of spaces of geodesic triangulations of more complicated surfaces. This is a joint work with Tianqi Wu and Xiaoping Zhu.
Date: Tuesday November 7, 2023	Speaker: Speaker: Yifan Jing, University of Oxford	Talk Title: Measure doubling for sets in locally compact groups
A central problem in additive combinatorics is to obtain a good lower bound for the size of the product set AB of two subsets A, B of a group G. In this talk, we will be focusing on the setting when G is a locally compact group, where G has a natural notion of size: the Haar measure. I will talk about some recent developments and their connections to analysis and geometry, including the resolution of the Inverse Kemperman problem, a non-abelian Brunn-Minkowski inequality, and the confirmation of a conjecture of Breuillard and Green. The talk is based on joint work with Chieu-Minh Tran and Ruixiang Zhang.
Date: Tuesday October 31, 2023	Speaker: Shivaramakrishna Pragada, SFU	Talk Title: Subdivision and adjacency spectra of graphs
In this talk, we investigate the asymptotic nature of graph spectra when some edges of a graph are subdivided sufficiently many times. We show that the eigenvalues of the sequences of graphs obtained by subdividing edges are Cauchy. As an application of the main result, we construct a bounded degree graph sequence with (approximate) high second eigenvalue multiplicity. This is a joint work with Hitesh Kumar, Bojan Mohar and Hanmeng Zhan.
Date: Tuesday October 24, 2023	Speaker: Thomas Pender, SFU	Talk Title: Balanced Weighing Matrices and Association Schemes
A weighing matrix is a square (-1,0,+1)-matrix with pairwise orthogonal rows such that each row has a constant number of non-zero entries. If upon taking the absolute values of all the entries of a weighing matrix, one obtains an incidence matrix for a symmetric balanced incomplete block design, we say that the weighing matrix is balanced. In this talk, we construct a new infinite family of balanced weighing matrices. We then show a novel characterization of balanced weighing matrices using association schemes. Finally, we explore analogous relations between more general classes of objects: balanced generalized weighing matrices, quasi-balanced weighing matrices. This is joint work with Dr. Hadi Kharaghani (University of Lethbridge) and Dr. Sho Suda (National Defense Academy of Japan).
Date: Tuesday October 3, 2023	Speaker: Shizhe Liang, Brandeis University	Talk Title: Grand-Schnyder woods and applications to drawing
In 1990, Schnyder showed that every plane triangulation admits a special partition of its inner edges into 3 trees, which are known as the Schnyder woods. Using this structure, Schnyder designed a straight-line grid drawing algorithm for triangulations. Since then, other structures and algorithms sharing the same flavor have been found by various authors. In this talk I will present a new construction, named grand-Schnyder woods, that simultaneously generalizes several of these structures (notably the original Schnyder woods, the regular edge labelings of Kant and He, and the d-Schnyder structures of Bernardi and Fusy). Precisely, a grand-Schnyder wood of parameter d is defined on a planar graph which has faces of degree at most d and non-facial cycles of length at least d. It is a set of d spanning forests crossing each other in a specific way. Grand-Schnyder woods of parameter d=4 inspires a general framework of drawing algorithms, which unifies several classical algorithms due to Kant and He, Fusy, Barriere & Huemer, and Bernardi & Fusy. This is a joint work with Olivier Bernardi and Eric Fusy.
Date: Tuesday September 26, 2023	Speaker: Alexander Clow, SFU	Talk Title: Injective and Oriented Colouring Graphs of Bounded Euler Genus
In this talk we consider the injective chromatic index and the oriented chromatic number of graphs with bounded Euler genus. In particular, we present our proofs of a tight (up to the choice of little o(1)) bound on the injective chromatic index in genus and that the oriented chromatic number is bounded polynomially in genus. The latter being a major improvement over the previous best upper bound which is exponential in genus. We conclude the talk by discussing directions for future study. Joint work with Peter Bradshaw (University of Illinois Urbana Champaign), and Jingwei Xu (University of Illinois Urbana Champaign).
Date: Tuesday September 19, 2023	Speaker: Maxwell Levit, SFU	Talk Title: 4-cycle-free covers of Cartesian products of cycles
In order to classify certain small generalized hexagons, Cohen and Tits proved that the d-dimensional hypercube has a unique 2-fold cover with no 4-cycles. I will discuss some reasons I find these covers interesting, including a close connection to Huang's recent proof of the Sensitivity conjecture from computer science. Then I will give a group-theoretic description of these covers which generalizes nicely to provide 4-cycle-free covers of Cartesian products of p-cycles.

11 Jan	Jesse Campion Loth, SFU	Permutations, probability and graph embeddings
Products of permutations are used to model problems in fields ranging from representation theory and algebraic geometry to topological graph theory. I will show how probability can be used to study such problems. I will start with some algebraic results on products of symmetric group conjugacy classes. I will then show how these results can be used to solve natural topological problems. The main focus of the talk will then be using these random techniques to study how graphs may be embedded on different surfaces.
18 Jan	Shuxing Li, SFU	Packings of Partial Difference Sets
As the underlying configuration behind many elegant finite structures, partial difference sets have been intensively studied in design theory, finite geometry, coding theory, and graph theory. In the first part of this talk, I will showcase that partial difference sets are indeed the fundamental structures explaining a series of “two-valued phenomena” in various mathematical branches. The second part concerns the construction of partial difference sets. Over the past three decades, there have been numerous constructions using very dif- ferent and delicate approaches. Surprisingly, we discovered a common frame- work unifying and extending many previous constructions. This is joint work with Jonathan Jedwab.
2 Feb	Prof. Dimitri Leemans, Université Libre de Bruxelles	The number of string C-groups of high rank
(Joint with Operations Research seminar) Abstract polytopes are a combinatorial generalisation of classical objects that were already studied by the greeks. They consist in posets satisfying some extra axioms. Their rank is roughly speaking the number of layers the poset has. When they have the highest level of symmetry (namely the automorphism group has one orbit on the set of maximal chains), they are called regular. One can then use string C-groups to study them. Indeed, string C-groups are in one-to-one correspondence with abstract regular polytopes. They are also smooth quotients of Coxeter groups. They consist in a pair $(G,S)$ where $G$ is a group and $S$ is a set of generating involutions satisfying a string property and an intersection property. The cardinality of the set $S$ is the rank of the string C-group. It corresponds to the rank of the associated polytope. In this talk, we will give the latest developments on the study of string C-groups of high rank. In particular, if $G$ is a transitive group of degree $n$ having a string C-group of rank $rgeq (n+3)/2$, work over the last twelve years permitted us to show that $G$ is necessarily the symmetric group $S_n$. We have just proven in the last months that if $n$ is large enough, up to isomorphism and duality, the number of string C-groups of rank $r$ for $S_n$ (with $rgeq (n+3)/2$) is the same as the number of string C-groups of rank $r+1$ for $S_{n+1}$. This result and the tools used in its proof, in particular the rank and degree extension, imply that if one knows the string C-groups of rank $(n+3)/2$ for $S_n$ with $n$ odd, one can construct from them all string C-groups of rank $(n+3)/2+k$ for $S_{n+k}$ for any positive integer $k$. The classification of the string C-groups of rank $rgeq (n+3)/2$ for $S_n$ is thus reduced to classifying string C-groups of rank $r$ for $S_{2r-3}$. A consequence of this result is the complete classification of all string C-groups of $S_n$ with rank $n-kappa$ for $kappain{1,ldots,6}$, when $ngeq 2kappa+3$, which extends previous known results. The number of string C-groups of rank $n-kappa$, with $ngeq 2kappa +3$, of this classification gives the following sequence of integers indexed by $kappa$ and starting at $kappa = 1$. $$Sigma{kappa}=(1,1,7,9,35,48).$$ This sequence of integers is new according to the On-Line Encyclopedia of Integer Sequences. Joint work with Peter J. Cameron (University of St Andrews) and Maria Elisa Fernandes (University of Aveiro)
8 Feb	Curtis Bright, University of Windsor	SAT Solvers, Isomorph-free Generation, and the Quest for the Minimum Kochen–Specker System
I will describe a new approach for exhaustively generating combinatorial objects by combining a satisfiability (SAT) solver with an isomorph-free exhaustive generation method such as orderly generation. The SAT solver is able to limit the search to objects that satisfy given criteria, while the isomorph-free generation method ensures that the objects are generated up to isomorphism. The combined search procedure performs orders-of-magnitude faster than a pure SAT or pure computer algebraic approach, as the SAT solver tailors the search to the object in question while the isomorph-free generation avoids duplication of work when the search space is highly symmetrical. As a motivating example, I will discuss how this approach can be applied to search for Kochen-Specker (KS) systems, an important combinatorial object arising in quantum physics. For example, a KS system is at the heart of the "KS Theorem" which intuitively states that a quantum observation cannot be understood as revealing a pre-existing value, and the "Free Will Theorem" which intuitively states that if humans have free will then so must quantum particles. Since 1990, the smallest known KS system in three dimensions has contained 31 vectors and much effort has been expended trying to find a KS system of smaller size. An exhaustive computer search in 2016 was able to disprove the existence of a KS system of 21 or fewer vectors. Our SAT and orderly generation approach is over 25,000 times faster than the previously used approach and has also ruled out the existence of a KS system with 22 vectors. This work is joint with Zhengyu Li and Vijay Ganesh (University of Waterloo).
15 Feb	Stephen Kobourov, University of Arizona	Contact Representation of Planar Graphs in 2D and 3D
In a proportional contact representation of a planar graph, each vertex is represented by a simple polygon with area proportional to a given weight, and edges are represented by adjacencies between the corresponding pairs of polygons. We show how to use Schnyder realizers and canonical orders for planar graphs to obtain different types of contact representations. Specifically, we describe an algorithm that constructs proportional contact representation for arbitrary planar graphs using 10-sided rectilinear polygons. We also describe a construction with 8-sided polygons, which is optimal in terms of polygonal complexity, as 8-sided polygons are sometimes necessary. In 3D vertices are represented by polytopes and edges by contacts between the corresponding polytopes contacts. We show that planar 3-trees have contact representations with cubes and proportional contact representations with boxes.
8 Mar	Harmony Zhan, SFU	On trees with extremal second largest eigenvalues
We will characterize members that maximize/minimize the second largest adjacency eigenvalue in certain families of trees. Joint work with H. Kumar, B. Mohar and S. Pragada.
15 Mar	Zhilin Ge, SFU	Hamiltonicity of Covering Graphs of Trees
We consider covering graphs obtained by lifting trees as voltage graphs over cyclic groups. We generalize a tool of Hell, Nishiyama, and Stacho, known as the billiard strategy, for constructing Hamiltonian cycles in the covering graphs of paths. The presentation will show that our extended tool can be used to provide new sufficient conditions for the Hamiltonicity of covering graphs of trees that are similar to those of Batagelj and Pisanski and of Hell, Nishiyama, and Stacho. Next, we focus specifically on covering graphs obtained from trees lifted as voltage graphs over cyclic groups Zp of large prime order p. The presentation will show that for a given reflexive tree T with random nonzero voltage labels from Zp on its edges, the corresponding lift is almost surely Hamiltonian for a large enough prime-ordered cyclic group Zp. Finally, we show that if a reflexive tree T is lifted over a group Zp of a large prime order, then for any assignment of nonzero elements of Zp to the edges of T, the corresponding cover of T has a large circumference.
5 Apr	Alexandra Wesolek, SFU	Cops and Robber Game on Surfaces of Constant Curvature
Recently, Mohar introduced a variant of the cops and robber game that is played on geodesic spaces. In this talk we discuss the rules of the game and strategies for the players when the game is played on compact surfaces of constant curvature, in particular surfaces of negative constant curvature. Joint work with Vesna Iršič and Bojan Mohar.
14 Apr	Daniel Panario, Carleton University	The Dynamics of Iterating Functions over Finite Fields
When we iterate functions over finite structures, there is an underlying natural functional graph. For a function $f$ over a finite field $\mathbb{F}_q$, this graph has $q$ nodes and a directed edge from vertex $a$ to vertex $b$ if and only if $f(a)=b$. It is well known, combinatorially, that functional graphs are sets of connected components, components are directed cycles of nodes, and each of these nodes is the root of a directed tree. The study of iterations of functions over a finite field and their corresponding functional graphs is a growing area ofresearch, in part due to their applications in cryptography and integer factorization methods like Pollard rho algorithm. Some functions over finite fields when iterated present strong symmetry properties. These symmetries allow mathematical proofs of some dynamical properties such as period and preperiod of a generic element, (average) ``rho length'' (number of iterations until a cycle is formed), number of connected components, cycle lengths, and permutational properties (including the cycle decomposition). We survey the main problems addressed in this area so far. We briefly comment on ongoing research about the functional graph of generalized cyclotomic mappings of finite fields. We study periodic points, cycle structure, and rooted trees attached to periodic points. We briefly comment on some open problems. Based on joint works with Alexander Bors (Carleton, Canada), Rodrigo Martins (UTFPR, Brazil), Claudio Qureshi (UdelaR, Uruguay), Lucas Reis (UFMG, Brazil) and Qiang (Steven) Wang (Carleton, Canada).

07 Oct Special Time: 11am	Bojan Mohar. Simon Fraser University	Beyond the Four-Color Theorem
The speaker will discuss the Four-Color Theorem and some of the outstanding problems left open after the proof.
14 Oct	Peter Bradshaw, Simon Fraser University	A small step forward in bipartite list coloring
The choosability of a graph G is the minimum value k such that G has a proper list coloring whenever each vertex of G has a list of k colors. Molloy showed that if G is a triangle-free graph of maximum degree d, then the choosability of G is at most (1 + o(1)) d / log d and that this bound is tight within a factor of roughly 2. It is widely believed that this upper bound can be greatly improved in the special case that G is bipartite, and Alon and Krivelevich conjectured an upper bound of O(log d) in this case. However, little progress has been made toward solving this conjecture, and it is currently unknown whether the optimal upper bound for bipartite graphs is different from the optimal upper bound for triangle-free graphs. In this talk, we will use a modification of a coupon-collector argument from Alon, Cambie, and Kang to show that every bipartite graph of maximum degree d has choosability at most 0.797 d / log d. Our result provides strong evidence that the list-coloring problem is fundamentally easier in bipartite graphs than in triangle-free graphs and hence takes an important step toward solving the conjecture of Alon and Krivelevich. (The content of this talk is part of ongoing work with Bojan Mohar and Ladislav Stacho.)
21 Oct	Harmony Zhan, Simon Fraser University	An introduction to discrete quantum walks
A discrete quantum walk is determined by a unitary matrix representation of a graph. In this talk, I will give an overview of different quantum walks, and show how the spectral information of the unitary matrix representation links properties of the walks to properties of the graphs. Part of this talk will be based on the book, Discrete Quantum Walks on Graphs and Digraphs, by Chris Godsil and me.
28 Oct	Zhouningxin Wang	Circular flows in mono-directed Eulerian signed graphs
In this talk, we consider analogs of Jaeger's circular flow conjecture and its dual Jaeger-Zhang conjecture in signed graphs. We will first give the notions of circular coloring and circular flow in signed graphs, and then show that every (6k-2)-edge-connected Eulerian signed graph admits a circular 4k/(2k-1)-flow and that every signed bipartite planar graph of negative-girth at least 6k-2 admits a circular 4k/(2k-1)-coloring (equivalently, admits a homomorphism to a negative cycle of length 2k). We also provide some recent results about circular flow indices of signed graphs with high edge-connectivities. This is based on joint work with Jiaao Li, Reza Naserasr, and Xuding Zhu.
4 Nov	Dong Ye, Middle Tennessee State University	Nowhere-zero Z3-flow and sign-circuit covering
In 1950s, Tutte discovered that the map-coloring problem on orientable surface could be interpreted as an integer flow problem. A classic result of Tutte states that a graph with a nowhere-zero Z3-flow has a circuit cover which covers every edge at most twice, and hence has a shortest circuit cover with length at most 4/3 of the total number of edges. For an orientable graphs on non-orientable surfaces, its dual is a signed graph. In this talk, we focus on nowhere-zero Z3-flows and circuit covers of signed graphs, extending Tutte’s result from graph to signed graphs. This is based on joint work with Jiaao Li and Yezhou Wu.
18 Nov	Daniel Neuen, SFU	Parameterized Algorithms for Graph Isomorphism - Decompositions via Regularity
In the first part of the talk, I will give an overview of recent advances on the graph isomorphism problem focusing on quasi-polynomial parameterized algorithms with a running time of the form n^{polylog(k)}, where k is some graph parameter. The first such algorithm was obtained for k being the maximum degree of the input graphs [FOCS 2018] by extending the group-theoretic tools of Babai's quasi-polynomial time isomorphism test [STOC 2016]. Subsequently, this result was generalized in a series of works to all graphs that exclude the complete graph on k vertices as a topological subgraph [SODA 2022]. In particular, this includes all graphs of Euler genus at most k-7 as well as all graphs of tree-width at most k-2. The key concept underlying this latest algorithm is the notion of t-CR-bounded graphs that serves as a gateway to combine the group-theoretic tools underlying the isomorphism test for bounded-degree graphs with decomposition-based approaches for graphs that exclude the complete graph on k vertices as a topological subgraph. In the second part of the talk, we explore the graph-theoretic side of the algorithm in more depth and discuss how regularity of the input graphs can be leveraged to obtain decompositions into parts with few symmetries that can then be handled by the group-theoretic graph isomorphism machinery.
25 Nov	Jonathan Jedwab, SFU	Constructions of difference sets in groups of order $4^d$
The study of difference sets lies at the intersection of design theory, coding theory, finite geometry, and algebraic number theory. The central problem is to determine which groups contain at least one difference set, and the richest existence results occur when the group order is a power of 4. The principal obstacles are that the number of such groups grows rapidly, and that the groups have such diverse structures that an overall theory seems out of reach. However, after a ten-year collaboration, this question was resolved completely for all 56,092 groups of order 256 using a new theoretical framework together with computational search. I shall describe the history of this problem, the methods used to resolve order 256, and the prospects for extending these techniques to order 1024.

18 Jan (ZOOM)	Jiaxi Nie, University of California San Diego	On Asymptotic Packing of Geometric Graphs
A set of geometric graphs is geometric-packable if it can be asymptotically packed into every sequence of drawings of the complete graph $K_n$. For example, the set of geometric triangles is geometric-packable due to the existence of Steiner Triple Systems. When G is the 4-cycle (or 4-cycle with a chord), we show that the set of plane drawings of G is geometric-packable. In contrast, the analogous statement is false when G is nearly any other planar Hamiltonian graph (with at most 3 possible exceptions). A convex geometric graph is convex-packable if it can be asymptotically packed into the convex drawings of the complete graphs. For each planar Hamiltonian graph G, we determine whether or not a plane G is convex-packable. Many of our proofs explicitly construct these packings; in these cases, the packings exhibit a symmetry that mirrors the vertex transitivity of $K_n$. here is the link. (ID: 668 0051 2140, password: Graph)
25 Jan	Kathryn Nurse, Simon Fraser University	Toward Bouchet’s conjecture on nowhere-zero flows in signed graphs.
I will present a work in progress. In 1954, Tutte proved flow-colouring duality and made a conjecture that every graph without a cut-edge has a nowhere-zero 5-flow. A parallel conjecture to this, but for signed graphs is Bouchet's conjecture (1983) that every signed graph without the obvious obstruction has a nowhere-zero 6-flow. We have been able to show that Bouchet's conjecture holds for 3-connected graphs when 6 is replaced with 8. This is joint with Matt DeVos and Robert Šámal.
01 Feb	Annie Raymond (UMass)	Tropicalization of graph profiles
The number of homomorphisms from a graph H to a graph G, denoted by hom(H;G), is the number of maps from V(H) to V(G) that yield a graph homomorphism, i.e., that map every edge of H to an edge of G. Given a fixed collection of finite simple graphs {H_1, ..., H_s}, the graph profile is the set of all vectors (hom(H_1; G), ..., hom(H_s; G)) as G varies over all graphs. Graph profiles essentially allow us to understand all polynomial inequalities in homomorphism numbers that are valid on all graphs. Profiles can be extremely complicated; for instance the full profile of any triple of connected graphs is not known. To simplify these objects, we introduce their tropicalization which we show is a closed convex cone that still captures interesting combinatorial information. We explicitly compute these tropicalizations for some sets of graphs, and relate the results to some questions in extremal graph theory. This is joint work with Greg Blekherman, Mohit Singh and Rekha Thomas.
08 Feb	Vesna Iršič (Simon Fraser University)	Winning vs. catching in the game of cops and robber on manifolds
Recently, Mohar introduced a variant of the cops and robber game that is played on geodesic spaces. The game combines properties of pursuit-evasion games with the classical cops and robber game played on graphs. In the game, cops win if they can get arbitrarily close to the robber. On the other hand, cops catch the robber if one of them occupies the same point as the robber. In this talk we will discuss several strategies for players in the game, and observe the difference between the number of cops needed to catch the robber and the number of cops needed to win the game. Joint work with Bojan Mohar and Alexandra Wesolek.
15 Mar	Bojan Mohar (Simon Fraser University)	Proper orientations and proper chromatic number
It is an interesting (and not entirely obvious) fact that every graph admits an orientation of its edges so that the outdegrees at vertices form a "coloring" (outdegrees of any two adjacent vertices are different). The proper chromatic number $\Vec{\chi}(G)$ of a graph $G$ is the minimum $k$ such that there exists an orientation of the edges of $G$ with all vertex-outdegrees at most $k$ and such that for any adjacent vertices, the outdegrees are different. Two major conjectures about the proper chromatic number are resolved. First it is shown, that $\Vec{\chi}(G)$ of any planar graph $G$ is bounded (in fact, it is at most 14). Secondly, it is shown that for every graph, $\Vec{\chi}(G)$ is at most $O(\frac{r\log r}{\log\log r})+\tfrac{1}{2}\MAD(G)$, where $r=\chi(G)$ is the usual chromatic number of the graph, and $\MAD(G)$ is the maximum average degree taken over all subgraphs of $G$. Several other related results are derived. Our proofs are based on a novel notion of fractional orientations. This is joint work with Yaobin Chen and Hehui Wu.
05 Apr	Zdenek Dvorak (Charles University)	3-coloring near-quadrangulations of the plane and the torus using flows (joint work with C. Bang, E. Heath, and B. Lidicky)
A graph drawn in a surface is a k-near-quadrangulation if the product of the lengths of its faces, except for 4-faces, is at most k. Near-quadrangulations play a fundamental role in the theory of 3-colorability of triangle-free graphs on surfaces. We show how the well-known duality between colorings and flows can be used to design * a 3-precoloring extension algorithm for k-near-quadrangulations of the plane, and * a 3-coloring algorithm for k-near-quadrangulations of the torus, with time complexity polynomial in k and the number of vertices.

09 Nov	Andrei Bulatov, Simon Fraser University	The complexity of counting homomorphisms
The Constraint Satisfaction Problem (CSP) provides a general framework for a wide range of combinatorial problems. The simplest way to state the problem is: Given relational structures (say, graphs) G and H, decide if there is a homomorphism from G to H. The counting version of the CSP, #CSP, asks for the number of such homomorphisms. In order to represent specific problems, the CSP is often restricted in various ways, in particular by fixing the target structure H. Thus, CSP(H) asks if, for a given structure G, there is a homomorphism from G to H. If H is a graph such a problem is often called H-Coloring. The counting versions #CSP(H), #H-Coloring of these problems are defined accordingly. In this talk we survey the developments in the study of the complexity of #CSP(H) that eventually led to a complete classification of such problems and their various generalizations. ZOOM Information: https://sfu.zoom.us/j/66800512140?pwd=N011dU5ZNThSWnVzMFBjYnBidDdmUT09 (ID: 668 0051 2140 Password: Graph)
16 Nov	Peter Bradshaw, Simon Fraser University	A rainbow connectivity threshold for random graph families
Given a family G of graphs on a common vertex set X, we say that G is rainbow connected if for every vertex pair u, v ∈ X, there exists a path from u to v that uses at most one edge from each graph of G. We consider the case that G contains s graphs, each sampled randomly from G(n, p), with n = \|X\| and p = c log n / sn , where c > 1 is a constant. We show that there exists a threshold of at most three consecutive integer values such that when s is greater than or equal to this threshold, G is a.a.s. rainbow connected, and when s is below this threshold, G is a.a.s. not rainbow connected. Joint work with Bojan Mohar.

19 May	Joy Morris, University of Lethbridge	Lexicographic products and wreath products
Abstract: I will present a history and overview of some of the work that has been done on the lexicographic product of graphs, and related generalisations. The focus of my talk will be on the automorphism groups of such graphs, and the relationship to the wreath product of permutation groups.
2 June	Xiang-dong Hou, University of South Florida	On the number of equivalence classes of boolean functions
Abstract: Two Boolean functions from $\Bbb F_2^n$ to $\Bbb F_2$ are called (affine) equivalent if one can be obtained from the other through an invertible affine transformation of the variables followed by an addition of an affine function. Most coding theoretic and cryptographic properties of Boolean functions are preserved under this equivalence. Let $N_n$ denote the number of equivalence classes of Boolean functions in $n$ variable. No explicit formula for $N_n$ is known. A long-standing open question by MacWilliams and Sloane asks for an asymptotic formula for $N_n$ as $n\to\infty$. Recently, we find a solution to this question. (Abstract in PDF)
8 June Tuesday @ 2 pm	Special session for the Round the World Relay in Combinatorics Fan Chung, University of California San Diego	Trees and forests in Green's functions of a graph
Abstract: The Green's functions of a graph are the pseudo inverses of the Laplacians and therefore are useful for solving many types of Laplace equations in discrete settings.In this talk, we will give combinatorial interpretations of Green's functions in terms of counting trees and forests in a graph. We will also mention several applications concerning the pagerank algorithms and the hitting time for random walks.
16 June	Akbar Rafiey, SFU	Fast and Private Submodular and k-Submodular Functions Maximization with Matroid Constraints
Abstract: The problem of maximizing nonnegative monotone submodular functions under a certain constraint has been intensively studied in the last decade, and a wide range of efficient approximation algorithms have been developed for this problem. Many machine learning problems, including data summarization and influence maximization, can be naturally modeled as the problem of maximizing monotone submodular functions. However, when such applications involve sensitive data about individuals, their privacy concerns should be addressed. In this paper, we study the problem of maximizing monotone submodular functions subject to matroid constraints in the framework of differential privacy. We provide (1−1/e)-approximation algorithm which improves upon the previous results in terms of approximation guarantee. This is done with an almost cubic number of function evaluations in our algorithm. Moreover, we study k-submodularity, a natural generalization of submodularity. We give the first 1/2-approximation algorithm that preserves differential privacy for maximizing monotone k-submodular functions subject to matroid constraints. The approximation ratio is asymptotically tight and is obtained with an almost linear number of function evaluations. Joint work with Yuichi Yoshida (http://proceedings.mlr.press/v119/rafiey20a.html)
23 June	Rebecca Patrias, University of St Thomas	Webs and tableau promotion
Abstract: We will start with an introduction to webs, standard Young tableau promotion, and the relationship between web rotation and tableau promotion. We will then discuss increasing tableaux---a K-theoretic analogue of SYT---and K-promotion. A big open question in this area deals with the order of K-promotion on increasing tableaux. We will talk about results in the 3-row case (in joint work with O. Pechenik) as well as current efforts to relate this problem to web promotion (joint with O. Pechenik, J. Striker, and J. Tymoczko).
30 June	Haggai Liu	Enumerative Properties of Cogrowth Series on Free products of Finite Groups
Abstract: Given a group G with a finite set of generators, S, it is natural to ask if the product of n generators from S evaluate to the identity. The enumerative version of this problem, known as the cogrowth problem, counts the number of such products and studies the associated counting sequence. Many cogrowth sequences are known. We focus on the free products of finite groups: Specifically, cyclic and dihedral groups. Such groups have the property that their cogrowth generating functions are algebraic functions, and thus, are solutions to implicit polynomial equations. Using algebraic elimination techniquesand free probability theory, we establish upper bounds on the degrees of the polynomial equations that they satisfy. This has implications for asymptotic enumeration, and makes it theoretically possible to determine the functions explicitly.
7 July	Christin Bibby, Louisiana State University	Combinatorics of orbit configuration spaces
Abstract: From a group action on a space, define a variant of the configuration space by insisting that no two points inhabit the same orbit. When the action is almost free, this "orbit configuration space" is the complement of an arrangement of subvarieties inside the cartesian product, and we use this structure to study its topology. We give an abstract combinatorial description of its poset of layers (connected components of intersections from the arrangement) which turns out to be of much independent interest as a generalization of partition and Dowling lattices. The close relationship to these classical posets is then exploited to give explicit cohomological calculations.
14 July	Yifan Jing, UIUC	Minimal and nearly minimal measure expansions in locally compact groups
Abstract: In 1964, Kemperman proved the following continuous nonabelian counterpart of the Cauchy-Davenport theorem: If G is a connected unimodular locally compact group with a left (and hence right) Haar measure \mu, A, B \subseteq G are nonempty and compact, and AB is their product set, then \mu(AB) \geq \min\{ \mu(A) + \mu(B), \mu(G)\}. I will present the recent joint works with Jinpeng An, Chieu-Minh Tran and Ruixiang Zhang where we determine the conditions for the equality to happen or nearly happen in the above inequality. Our results and methods answer several questions by Griesmer, Henstock, Hrushovski, Kemperman, Macbeath, McCrudden, and Tao.
21 July	Sophie Spirkl, Waterloo	The complexity of list-5-colouring with a forbidden induced sugbraph
Abstract: A k-list-assignment for a graph G is a function L from V(G) to the set of subsets of {1,…,k}. The list-k-colouring problem asks, given G and a k-list-assignment L, is there a colouring f of G with f(v) in L(v) for all v in V(G)? This generalizes the k-colouring problem (where we use L(v)={1,…,k} for every vertex). For k at least 3, both k-colouring and list-k-colouring are NP-hard, which motivates studying the complexity of these problems when the input is restricted to H-free graphs (for a fixed H, excluded as an induced subgraph). I will present an algorithm for list-5-colouring restricted to H-free graphs for all H which consist of connected components each of which is a 3-vertex path. This, together with previous results, gives a complete answer to the question, “for which H is list-5-colouring restricted to H-free graphs NP-hard? Joint work with Sepehr Hajebi and Yanjia Li.
28 July	Anurag Bishnoi, TU Delft	The minimum degree of minimal Ramsey graphs for cliques
Abstract: In this talk we will present a new upper bound of $s_r(K_t) = O(t^5r^{5/2})$ on the Ramsey parameter $s_r(K_t)$ introduced by Burr, Erd\H{o}s and Lov\'{a}sz in 1976, which is defined as the smallest minimum degree of a graph $G$ such that any $r-colouring of the edges of $G$ contains a monochromatic $K_t$, whereas no proper subgraph of $G$ has this property. This improves the previous upper bound of $s_r(K_t) = O(t^6r^3)$ proved by Fox et al. The construction used in our proof relies on a group theoretic model of generalised quadrangles introduced by Kantor in 1980. This is joint work with John Bamberg and Thomas Lesgourgues (https://arxiv.org/abs/2008.02474).

20 January	Peter Bradshaw, SFU	Flexible list colorings in graphs of bounded degree
^{Abstract: For a given $\epsilon > 0$, we say that a graph $G$ is $\epsilon$-flexibly $k$-choosable if the following holds: for any assignment $L$ of lists of size $k$ on $V(G)$, if a preferred color is requested at any set $R$ of vertices, then at least $\epsilon \|R\|$ of these requests are satisfied by some $L$-coloring. We characterize the graphs of maximum degree $\Delta$ that are $\epsilon$-flexibly $\Delta$-choosable for some $\epsilon = \epsilon(\Delta) > 0$, which answers a question of Dvořák, Norin, and Postle [List coloring with requests, JGT 2019]. In particular, we show that for any $\Delta\geq 3$, any graph of maximum degree $\Delta$ that is not isomorphic to $K_{\Delta+1}$ is $\frac{1}{2 \Delta^3}$-flexibly $\Delta$-choosable. This is joint work with Ladislav Stacho and Tomáš Masařík.}
27 January 10am Start	Ambat Vijayakumar, Cochin University of Science and Technology, India	Distance spectrum of graphs - some recent studies
^{Abstract: The distance matrix of a connected graph G is an n × n matrix $D(G) = [d_{ij}]$, where $d_{ij}$ is the distance between the vertices $v_i$ and $v_j$ . The distance spectrum of G is ${\lambda_1, . . . , \lambda_n}$, where $\lambda_i$'s are the eigenvalues of D(G). The distance energy of G is defined by $E_D(G) = \sum_{i=1}^n \|\lambda_i\|$. The distance matrix, dstance eigenvalue, and distance energy of a connected graph have been studied intensively in literature. We discuss a new problem of how the distance energy changes when an edge is deleted. Similar problem for adjacency energy of a graph was studied by Day and So. It turns out that the results for distance energy change and adjacency energy change are quite different. It follows that, for any connected graph with a unique positive eigenvalue, the deletion of any edge increases the distance energy provided that the resulting graph is still connected. We prove that the distance energy of a complete bipartite graph is always increased when an edge is deleted even though it has two positive distance eigenvalues. Also, we give a set of examples of connected graph whose distance energy decreases when a specific edge is deleted. This is a joint work with G. Indulal and A. Varghese.}
3 February	Damanvir Binner, SFU	Proofs of Berkovich and Uncu conjectures regarding partition inequalities
^{Abstract: We use techniques from elementary number theory (such as Frobenius Numbers) to combinatorially prove four recent conjectures of Berkovich and Uncu (Ann. Comb. 23 (2019) 263–284) regarding inequalities between the sizes of two closely related sets consisting of integer partitions whose parts lie in the interval {s, . . . , L+s}. Further restrictions are placed on the sets by specifying impermissible parts as well as a minimum part. (Joint work with A. Rattan)}
10 February	Pengyu Liu, SFU	Polynomial tree-based analysis
^{Abstract: We define a graph polynomial for unlabeled trees and show that the polynomial is a complete isomorphism invariant for unlabeled trees. We also demonstrate some applications of the tree distinguishing polynomial in tree-based analysis with examples in life sciences and linguistics.}
17 February Reading Break

24 February	Harmony Zhan, York University	Arc-reversal quantum walks
^{Abstract: A discrete quantum walk usually takes place on the arcs of a graph. Each step of the walk consists of two operations - a "coin flip" and an "arc shift". In this talk, we will consider quantum walks where the "arc shift" simply reverses each arc. Using different coins, I will show the connection of these walks to various graph structures, such as line digraphs, covers and embeddings.}
2 March TuesDAY @ 11:30-13:30	SpeCial Session: Nancy Ann Neudauer, Pacific University	Being Together and Better: Building, Belonging,and Giving Back to Mathematical Communities
^{Abstract: What have I learned about building mathematical communities over the last twenty years, initially in North America and more recently in Africa and South America? What does “belonging” mean to those who are already part of a mathematical community, and how can we provide the sort of welcome we once received (or would have liked to receive)? What constructive actions can we take to help mathematics students transition to a research-based academic career, and how might these differ when the students are located in developing countries without access to established infrastructure? Dr. Neudauer will discuss these and other questions in a special session of the SFU Discrete Mathematics Seminar, intended for students and faculty at all career stages. She will share stories and insights gained from decades of service to the mathematical community as a researcher, teacher, mentor, and organizer.}
10 March	Zeying Wang, Michigan Tech	Partial difference sets in abelian groups
^{Abstract: Recently we proved a theorem for strongly regular graphs that provides numerical restrictions on the number of fixed vertices and the number of vertices mapped to adjacent vertices under an automorphism. We then used this result to develop some new techniques to study regular partial difference sets in abelian groups. We have proved several non-existence results and classification results of partial difference sets in abelian groups. Also we completely answered the question "For which odd positive integer $v>1$, can we find a Paley type partial difference set in an abelian group of order $v$?", a classical question from the 1990s. In this talk I plan to give an overview of these results.}
17 March	Riana Roux, Stellenbosch University	A localization game on graphs
^{Abstract: The classic game of cops and robbers on a graph is a pursuit-evasion game introduced in the late 1970's. In the original game the cops and robber take turns to either stay at their present vertex or move to an adjacent vertex. The cops win the game if at any moment a cop occupies the same vertex as the robber. In this talk we will consider a game where the robber is invisible and where the cop only has to locate the robber, that is, the cop does not have to be on the same vertex as the robber to win. This game is called the localization game and consist of two players: a team of k cops and a robber. The game starts with a robber occupying some vertex on the underlying graph. The cop team then probes k vertices and receives the distance of the robber to each of these probed vertices. If the cop can deduce the robber’s location from this information he wins, otherwise the robber may move to an adjacent vertex and the cop again probes k vertices. If the cop can locate the robber in a finite number of turns, the cop wins. Otherwise, the robber wins. The localization number of a graph is the smallest k for which the cop wins. During this talk we will discuss some basic results regarding the localization number, ideas around forbidden subgraphs and the game played on specific graphs classes.}
24 March	Daniel Panario, Carleton University	Finite fields in combinatorial arrays: constructions and applications
^{Abstract: We discuss constructions based on finite fields of three types of combinatorial arrays: orthogonal, covering and ordered orthogonal arrays. Other combinatorial arrays have been similarly considered in the literature, including permutation arrays, frequency permutation arrays, hypercubes and Costas arrays. In this talk, for each of these arrays, we briefly explain constructions based on finite fields, and main applications. An Orthogonal Array (OA) of strength $t$ on $v$ symbols is an array with the property that, for every $t$-combination of column vectors, every one of the possible $v^t$ $t$-tuples of symbols appears as a row ``exactly'' once in the subarray defined by these column vectors. OAs have been applied in coding theory and in cryptography. They are equivalent to MDS (maximum distance separable) codes where the strength of the OA is closely related to the minimum distance of the code. OAs are also closely related to secret sharing in cryptography. An Ordered Orthogonal Arrays (OOA) is a generalization of OAs where the coverage property applies to some selected columns of the array. Similarly, a Covering Array of strength $t$ on $v$ symbols is an array with the property that, for every $t$-combination of column vectors, every one of the possible $v^t$ $t$-tuples of symbols appears as a row ``at least'' once in the subarray defined by these column vectors. Covering arrays are used to reduce the number of tests in application areas such as software testing and group testing. A common theme on several recent constructions of these arrays is the use of linear feedback shift register (LFSR) sequences and finite fields. Arrays whose rows are cyclic shifts of a sequence over a finite field possess many combinatorial properties. They have been used to build arrays attaining a high number of $t$-subsets of columns with the desired ``coverage property''. We provide examples of applications of these combinatorial arrays.}talk slides
31 March	Tomáš Masařík, SFU	Optimal discretization is fixed-parameter tractable
^{Abstract: Given two disjoint sets W_1 and W_2 of points in the plane, the Optimal Discretization problem asks for the minimum size of a family of horizontal and vertical lines that separate W_1 from W_2, that is, in every region into which the lines partition the plane there are either only points of W_1, or only points of W_2, or the region is empty. Equivalently, Optimal Discretization can be phrased as a task of discretizing continuous variables: we would like to discretize the range of x-coordinates and the range of y-coordinates into as few segments as possible, maintaining that no pair of points from W_1×W_2 are projected onto the same pair of segments under this discretization. We provide a fixed-parameter algorithm for the problem, parameterized by the number of lines in the solution. Our algorithm works in time 2^O(k^2 log(k))n^O(1), where k is the bound on the number of lines to find and n is the number of points in the input. Our result answers in positive a question of Bonnet, Giannopolous, and Lampis [IPEC 2017] and of Froese [PhD thesis, 2018] and is in contrast with the knownintractability of two closely related generalizations: the Rectangle Stabbing problem and the generalization in which the selected lines are not required to be axis-parallel. Joint work with: Stefan Kratsch, Irene Muzi, Marcin Pilipczuk, Manuel Sorge}
7 April	Amy Wiebe, Free University of Berlin	A combinatorial approach to Minkowski tensors of polytopes
^{Abstract: Intrinsic volumes of a convex body provide scalar data (volume, surface area, Euler characteristic, etc. ) about the geometry of a convex body independent of the ambient space. Minkowski tensors are the tensor-valued generalization of intrinsic volumes. They provide more complex geometric information about a convex body, such as its shape, orientation, and more. Minkowski volume tensors are closely linked to the moments of the uniform distribution on a convex body, and a rational generating function for these moments allows us to extract the tensors symbolically. In this talk, we explain this connection and show that it can be extended to the setting of Minkowski "surface tensors". We demonstrate how this generating function approach allows us to give an explicit formula for these surface tensors in the case of simplicial polytopes. No prior knowledge of Minkowski tensors will be assumed. This is a joint work with Büşra Sert and Niklas Livchitz}
14 April	Carolyn Chun, United States Navel Academy	Graphs and binary matroids whose odd circuits all have size three or five
^{Abstract: Graphs with no odd circuits are well understood. In 1992, Maffray characterized all graphs whose odd circuits only have size three. Oxley and Wetzler generalized this result to binary matroids in 2016. We give a complete characterization of all binary matroids whose odd circuits all have size three or five, as well as the graphs whose odd cycles all have size three or five. This talk is based on joint work with Oxley and Wetzler.}talk slides

16 September	Marthe Bonamy, LaBRI	On Vizing's edge colouring question
In his 1965 seminal paper on edge colouring, Vizing proved that a (Delta+1)-edge colouring can be reached from any given proper edge colouring through a series of Kempe changes, where Delta is the maximum degree of the graph. He concludes the paper with the following question: can an optimal edge colouring be reached from any given proper edge colouring through a series of Kempe changes? In other words, if the graph is Delta-edge-colourable, can we always reach a Delta-edge-colouring? If true, this would imply a more recent conjecture of Mohar (2006) that in any graph, all (Delta+2)-edge-colourings are equivalent up to a series of Kempe changes. We discuss recent progress around these questions.
23 September	Carla Groenland, University of Oxford	Asymptotic dimension of graph classes
I will attempt to give some intuition for asymptotic dimension of graph classes, starting from the definition and giving the connection to clustered colouring and weak-diameter network decompositions. I will sketch the ideas behind some of our results, e.g. that the class of planar graphs has asymptotic dimension 2. Asymptotic dimension was introduced by Gromov in 1993 within geometric group theory and in the graph setting makes sense for infinite graphs or graph classes. Joint work with M. Bonamy, N. Bousquet, L. Esperet, F. Pirot, and A. Scott.
30 September	Karen Yeats, University of Waterloo	Chains of hourglasses and the graph theory of their Feynman integrals
I will give a set up for understanding the graph theory of Feynman integrals that I have been using over the last number of years. It is all very combinatorial and does not require any understanding of physics or analysis. Then I will explain about some joint work with Oliver Schnetz where we are able to complete a generalized denominator reduction on a particular family of graphs and from this calculate an arithmetic graph invariant on them known as the c_2 invariant as well as understand properties of their Feynman integrals.
7 October	Shanise Walker, University of Wisconsin Eau Claire	The Game of Cycles
The Game of Cycles was introduced by Francis Su in his book titled Mathematics for Human Flourishing (2020). The game is played on a simple connected planar graph together with its bounded cells, and players take turns marking edges with arrows according to a sink-source rule. The object of the game is to produce a cycle cell–a cell surrounded by arrows all cycling in one direction–or to make the last possible move. In this talk, we explore the two-player game for various classes of graphs and determine who has a winning strategy.
14 October	Laura Escobar, Washington University in St. Louis	Which Schubert varieties are Hessenberg varieties?
Schubert varieties are subvarieties of the flag variety whose geometry can be understood using the combinatorics of permutations. Hessenberg varieties are also subvarieties of the flag variety with connections to both algebraic combinatorics and representation theory. After introducing and motivating these varieties, I will discuss joint work with Martha Precup and John Shareshian in which we investigate which Schubert varieties in the full flag variety are Hessenberg varieties.
21 October	Inês Rodrigues, University of Lisbon	An action of the cactus group on the shifted tableau crystal
The shifted tableau crystal is a crystal-like structure on shifted semistandard tableaux, introduced by Gillespie, Levinson and Purboo (2017). We define on this structure a shifted version of the crystal reflection operators, originally introduced by Lascoux and Schützenberger on type A crystals, which coincide with the restrictions of the shifted Schützenberger involution on primed intervals of two adjacent letters. Unlike type A crystal they not yield an action of the symmetric group. Following a similar approach as Halacheva (2016), we then exhibit a natural action of the cactus group on the shifted tableau crystal, realized by the restrictions of the shifted Schützenberger involution to all primed intervals of the underlying crystal alphabet, including, in particular, the crystal reflection operators.
28 October	Jana Novotná, University of Warsaw/Charles University	Clique-width of Atoms and Coloring for Hereditary Graphs
Clique-width is a well-established graph parameter. Many NP-complete graph problems are polynomially solvable on graph classes of bounded clique-width. Moreover, several of these problems are polynomially solvable on a hereditary graph class G if they are so on the atoms (graphs with no clique cut-set) of G. For these reasons, the boundedness of clique-width has been investigated for many graph classes. In this talk, we consider graph classes defined by one or two forbidden induced subgraphs. We discuss the coloring problem and the boundedness of clique-width of their atoms.
4 November	Karen Meagher, University of Regina	Erdős–Ko–Rado Theorems for Groups
The first half of this talk will be a gentle introduction to the Erdős–Ko–Rado (EKR) Theorem. This is a theorem that determines the size and structure of the largest collection of intersecting sets. It has become a cornerstone of extremal set theory and has been extended to many other objects. I will show how this result can be proven using techniques from algebraic graph theory. In the second half of this talk I will give more details about extensions of the EKR theorem to permutations. Two permutations are intersecting if they both map some i to the same point (so σ and π are intersecting if and only if σ⁻¹π has a fixed point). In 1977, Deza and Frankl proved that the size of a set of intersecting permutations is at most (n-1)!. It wasn't until 2003 that the structure of sets of intersecting permutations that meet this bound was determined. For any group action, we can ask what is the size and structure of the largest set of intersecting permutations. I will show some recent results for different groups, with a focus on 2-transitive groups.
12 November 2:30-3:20pm	Patricia Hersh, University of Oregon	Posets arising as 1-skeleta of simple polytopes, the nonrevisiting path conjecture, and poset topology
Held jointly with the SFU Operations Research Seminar Note the nonstandard time and day of the week
Abstract: Given a polytope P and a nonzero vector c, the question of which point in P has largest inner product (or smallest inner product) with c is the main goal of linear programming in operations research. Key to efficiency questions regarding linear programming is the directed graph G(P,c) on the 1-skeleton of P obtained by directing each edge e(u,v) from u to v for c(u) < c(v) and in particular the diameter of G(P,c). We will explore the question of finding sufficient conditions on P and c to guarantee that no directed path ever revisits any polytope face that it has left; this is enough to ensure that linear programming is efficient under all possible choices of pivot rule. It turns out that poset-theoretic techniques and poset topology can help shed light on this question. In discussing work on this question, we will provide background along the way.
18 November	Natasha Morrison, University of Victoria	Hypergraph Lagrangians: Extremal Combinatorics and the Frankl-Füredi conjecture
In this talk we explore the origins of hypergraph Lagrangians and their applications to classical problems in extremal combinatorics. We then focus on the Frankl-Füredi conjecture, which concerns the maximal value of the Lagrangian of an r-uniform hypergraph with m edges. In particular, we show that the conjecture is false in general, but holds for all sufficiently large 3-uniform hypergraphs. This talk is based on joint work with Vytautas Gruslys and Shoham Letzter.
25 November	Amélie Trotignon, Johannes Kepler University	Discrete harmonic functions in the three-quarter plane
Harmonic functions play an important role in probability theory and are strongly related to the enumeration of walks. Doob h-transform is a way to build conditioned random processes in cones from a random process and a positive harmonic function vanishing on the boundary of the cone. Finding positive harmonic functions for random processes is therefore a natural objective in the study of confined random walks. There are very few ways to compute discrete harmonic functions. In this talk we are interested in positive discrete harmonic functions with Dirichlet conditions in three quadrants. Whereas planar lattice (random) walks in the quadrant have been well studied, the case of walks avoiding a quadrant has been developed lately. We extend the method in the quarter plane – resolution of a functional equation via boundary value problem using a conformal mapping – to the three-quarter plane applying the strategy of splitting the domain into two symmetric convex cones. We obtain a simple explicit expression for the algebraic generating function of harmonic functions associated to random walks avoiding a quadrant.
2 December	Sofiat Olaosebikan, University of Glashow	Algorithmics of the Student-Project Allocation problem
Matching problems are all around us – they arise when we try to find the “best” allocation between two sets of entities. For example, matching transplant patients with organ donors, allocating junior doctors to hospitals, students to projects, and wireless networks to users. This class of problems was first studied in 1962 by Gale and Shapley, and its founding researchers were awarded a Nobel prize in 2012. My talk is centred around the Student-Project Allocation problem (SPA), which, in its natural form, involves finding a many-to-one matching of students to projects offered by lecturers, based on student preferences over projects and the maximum number of students that each project and lecturer can accommodate. During my PhD, I explored SPA with lecturer preferences over Projects (SPA-P), SPA with lecturer preferences over Students (SPA-S), and SPA-S with Ties (SPA-ST). The solution concept that we seek in this context is a “stable matching” – a matching that ensures that no student and lecturer who are not paired together would rather be assigned to each other than remain with their current assignees. In my talk, I will present the structural and algorithmic results arising from my research on finding stable matchings in the problem models mentioned above.
8 December	Aparna Lakshmanan, St. Xavier's College	On Leech Labeling
Note the nonstandard day of the week
A tree is called a Leech tree, if one can assign positive integer edge weights such that the nC2 path weights are exactly 1, 2,..., nC2, where the weight of a path p, w(P) is the sum of weights of all its edges and such a labeling is called a Leech labeling. Motivated by a problem in electrical engineering, the concept was introduced by John Leech in 1975 and he found five examples which are the only known Leech trees till date. We have introduce a new parameter called Leech index which gives a measure of how close a tree is towards being a Leech tree. Let f : E → {1, 2, 3, . . . } be an edge labeling of T such that both f and the corresponding path weight function w are injective. Let S denote the set of all weights of the paths in T. Let kf be the positive integer such that {1, 2, 3, . . . , kf } ⊆ S and kf + 1 ∈/ S. Then k(T) = max kf , where the maximum is taken over all such edge labelings f is called the Leech index of T. We determine the Leech index of several families of trees and obtain bounds for this parameter. The concept of Leech trees has been extended to general graphs also and studies are going in that direction too.

	All the remaining sessions cancelled due to the coronavirus disease (COVID-19)
10 March	David Wood, Monash University	Graph and Poset Dimension Parameters
3 March	Alexander Pott, Otto von Guericke University Magdeburg	Vanishing flats: a combinatorial viewpoint on the nonlinearity of functions
25 Feburary	Mehdi Salimi, SFU	Pursuit-Evasion Games
18 Feburary	Jason Schoeters, University of Bordeaux	Temporal cliques admit sparse spanners
11 Feburary	Timothy Chan, Monash and Warwick	Matroid branch-depth and integer programming
4 Feburary	(Cancelled due to campus snowfall closure)
28 January	Joris van der Hoeven, CNRS (Distinguished PIMS Discrete Math talk)	Integer multiplication in time O(n log n)
21 January	Joris van der Hoeven, CNRS	Historic algorithms for integer multiplication
14 January	Kevin Halasz, SFU	Near transversals of latin squares

03 December	Stephanie van Willigenburg, UBC	The e-positivity of chromatic symmetric functions
26 November	Peter Horak, University of Washington	Extremal problems on cycles in graphs
19 November	Thaís Bardini, SFU	Fault tolerance in cryptographic applications using cover-free families
12 November	Brin Chan, UBC	A generalization of balanced tableaux and matching problems with unique solutions
05 November	Pavol Hell, SFU	Graphs with possible loops
29 October	Kathryn Nurse, SFU	Maximum Directed Linear Arrangement
22 October	Robert Samal, Charles University	The binary paint shop problem (aka necklace splitting problem)
15 October	Farid Aliniaeifard, UBC	Combinatorial Hopf algebras and representation theory
08 October	Rikke Marie Langhede, DTU	The weak group connectivity number versus the group connectivity number
01 October	Petr Lisonek, SFU	Maximally non-associative quasigroups
24 September	Gary MacGillivray, University of Victoria	Switching (m,n)-edge-coloured graphs
17 September	Gonzalez Hermosillo de la Maza, SFU	Active cops and robbers
10 September	Peter Bradshaw, SFU	Graphs with high cop number
03 September	Curtis Bright, University of Waterloo	SAT Solving with Computer Algebra: A Powerful Combinatorial Search Method

23 April	Amarpreet Rattan, SFU	Parking functions and factorizations of the full cycle.
16 April	Igor Shinkar, SFU	On (almost) Lipschitz mappings on the discrete hypercube
9 April	Peter Bradshaw, SFU	Cops and Robbers on Cayley Graphs
2 April	Nike Dattani, Harvard-Smithsonian Center for Astrophysics	Solving mathematical and machine learning problems on quantum computers
26 March	Peter Horak, University of Washington	Old and New Conjectures in Tilings
19 March	Bojan Mohar, SFU	Toward a Theory of Crossing-Critical Graphs, Part I

4 December	Michael Monagan, SFU	How to factor a multivariate polynomial
20 November	Mahdieh Malekian, SFU	Excluding small graphs as immersion
13 November	Bojan Mohar, SFU	50 Years of the Ringel-Youngs Map Colour Theorem
23 October	Jonathan Jedwab, SFU	New Constructions of Difference Matrices
16 October	Leionid Chindelevitch, SFU	Metabolic networks: discrete geometry and optimization
9 October	Matt DeVos, SFU	On the colourful world of Ron Aharoni

7 August	Fuji Zhang	Spectral dynamics of graph eigenvalues
31 July	Sebastian Gonzalez, SFU
10 July	Yifan Jing, SFU	On the genus of complete 3-uniform hypergraphs
26 June	Andrew Berget, Western Washington U	Internal Zonotopal Algebras of Reflection Arrangements
5 June	Cedric Chauve, SFU	MaxTiC, ranking species trees using network constraints
29 May	Amy Wiebe, University of Washington	The Slack Realization Space of a Polytope
22 May	James Davis, University of Richmond	Apple vs Samsung: a Mathematical Battle
15 May	Haiyan Chen	On the sandpile group of a polygon flower

12 April	Jephian Lin	The zero forcing process and the strong Arnold property
31 March, Seattle	Sylvie Corteel, Kevin Purbhoo, Steph van Willigenburg, Matjaž Konvalinka	Pacific NW Combinatorics day, UWash, Seattle
24-25 Mar, Victoria		2018 Coast Combinatorics Conference University of Victoria, Victoria, BC
20 Mar	Ryan Hayward, Alberta	Recent results on Hex
13 Mar	Seyyed Aliasghar Hosseini, Simon Fraser University	Cops and Robbers on Graphs of Bounded Diameter
6 Mar	Stephen Melczer, UPenn	Eventual Positivity of Rational Function Coefficients
27 Feb	Amanda Montejano Cantoral, Universidad Nacional Autónoma de México	Forced chromatic patterns in 2-colorings of the complete graph
20 Feb	(cancellation)
13 Feb	No seminar (Reading week)
6 Feb	Philippe Nadeau, Lyon/CNRS	Combinatorics of the category of noncrossing partitions
30 Jan	Samantha Dahlberg, UBC	The e-positivity of claw-contractible-free graphs
23 Jan	Karen Meagher, Regina	Approaches to the Erdős-Ko-Rado Theorem
16 Jan	Jessica McDonald, Auburn	Edge-colouring planar graphs with precoloured edges

Dec 5	Karen Yeats C&O Waterloo	A little progress on the completion invariance of the c₂ invariant
Nov 28	Koen van Greevenbroek Math, SFU	Contracted Difference Matrices
Nov 21	Tony Huynh Université Libre de Bruxelles	A tight Erdős-Pósa function for wheel minors
7 Nov	Leonid Chindelevitch SFU	Finding the rank median of three genomes
24 Oct	Ross McConnell Colorado State	Planar and Outerplanar Graphs
17 Oct	Ron Aharoni Israel Institute of Technology	Cooperative colorings
10 Oct	Sheila Sundaram Pierrepont School	Variations on the S_n-module Lie_n: results and conjecture
3 Oct	Amanda Montejano Universidad Nacional Autónoma de México	Zero-sum over Z
26 Sept	Jehanne Dousse Universität Zürich	Refinement of partition identities
19 Sept	Moshe Rosenfeld U of Washington	Starters what you know or don't care to know: old and new open problems
12 Sept	Amelie Trotignon SFU	Simple Walks in the Three Quarter Plane

29 Aug	David Wood Monash University	Monotone expanders and applications
08 Aug	Aniket Mane SFU	A tractable variant of the Single Cut or Join distance with duplicated genes
08 Aug	Abhinav Shantanam SFU	Hadwiger Number of Complements of Kneser Graphs with \alpha = 2
01 Aug	Dirk Nowotka Kiel University	k-abelian pattern matching
25 July	Cesar Hernandez Cruz Universidad National Automate de Mexico	k-quasi-transitive digraphs of large diameter
25 July	Samuel Simon SFU	Linking Systems of Difference Sets
18 July	Shuxing Li SFU	Construction and nonexistence of strong external difference family
11 July	Julian Sahasrabudhe U of Cambridge	Exponential Patterns in Arithmetic Ramsey Theory
04 July	Avi Kulkarni SFU	Reading graphs into SAGE from an IPE diagram
04 July	Sebastian Gonzalez Hermosillo de la Maza SFU	Proper Orientations of Planar Bipartite Graphs
06 Jun	Arnaud Casteigts, LaBRI, U de Bordeaux	Robustness in Highly Dynamic Networks
30 May	Joao Meidanis, U of Campinas, Brazil	Genome Matrices and the Median Problem
23 May	Stephen Melczer, U of Waterloo	Effective Analytic Combinatorics in Several Variables
16 May	Mark Kayll, U of Montana	Derangements through the years: 1708--1996--2014

18 April	Julian Sahasrabudhe, U of Cambridge	Exponential Patterns in Arithmetic Ramsey Theory
28 Mar	Jonathan Jedwab, SFU	Costas cubes
21 Mar	Pavol Hell, SFU	Obstruction Certificates for Geometrically Defined Graph (and Digraph) Classes
14 Mar	David Rowe, Universitat Mainz	*This Talk Will Begin At 2pm* The Role of Configurations in Classical Algebraic Geometry
7 Mar	Peter Wu, SFU	On maximal size of 2-weakly compatible split system
28 Feb	Sean Griffin, U of Washington	Schur positivity and labeled binary trees
21 Feb	Bojan Mohar, SFU	The Crossing Number of the Cone of the Graph
7 Feb	Fiachra Knox, SFU	Majority Colourings of Digraphs
31 Jan	Luis Goddyn, SFU	Parity Trackles on Surfaces and Perturbations of Bilinear Forms

Date	Speaker, Affiliation	Talk Title
20 Sep	Tony Huynh, U Libre de Bruxelles	The matroid secretary problem for minor-closed classes and random matroids
18 Oct	Marni Mishna, SFU	Universality classes for weighted lattice paths: where probability and ACSV meet
25 Oct	Jørgen Bang-Jensen, U of Southern Denmark	Connectivity properties of tournaments and semicomplete digraphs
1 Nov	Seyyed Aliasghar Hosseini, SFU	Cops and Robbers on Oriented Grids I
8 Nov	Sebastian Gonzalez, SFU	Cops and Robbers on Oriented Grids II
15 Nov	Samantha Dahlberg, UBC	Pattern Avoidance in Restricted Growth Functions
22 Nov	Steffen Borgwardt, U of Colorado	The Hirsch Conjecture is true for Network-Flow Polytopes
29 Nov	Justin Chan, SFU	Suitable Cores And Arrays
*Please Note Date/Location: Talk on December 5th will be held in the IRMACS Theatre on MONDAY, Dec 5th
5 Dec	Fan Chung Graham, U of California, San Diego	Sequences: random, structured or something in between?

12 Jan	Rodolphe Garbit Université d'Angers	On the exit time from a cone for random walks with drift
19 Jan	Suil O Simon Fraser	Interlacing families and their application to digraphs
26 Jan IRMACS Theatre 1:00pm (12:00 reception)	Kilian Raschel CNRS / U François Rabelais - Tours	Counting quadrant walks via Tutte's invariants and transformation theory of elliptic functions
2 Feb	Stefan Hannie Simon Fraser	Immersion of 2-regular digraphs
9 Feb	Reading break	CANCELLED
16 Feb	Pierre Tarrago Saarlandes University	Categories of two-colored non-crossing partitions
23 Feb	—	CANCELLED
1 Mar	Cedric Chauve Simon Fraser	Counting, generating and sampling tree alignments
8 Mar	Julien Courtiel UBC	Terminal chords in connected diagrams chords
15 Mar	Claudia Linhares Sales UF Ceará	B-continuity of lexicographic product of graphs
22 Mar	Alejandro Morales UCLA	Hook Formulas for Skew Shapes
22 Mar (2nd talk) 3:00pm	Barnaby Martin Middlesex University London	The packing chromatic number of the infinite square lattice
29 Mar	Fiachra Knox Simon Fraser	Ramanujan gap for the norm of adjacency operators
5 Apr	---	PIMS-SFU Colloquium: Anthony Várilly-Alvarado in IRMACS Theatre

12 May	Nicholas Beaton U. Saskatchewan	Solvable self-avoiding walk and polygon models with large growth rates
19 May	Matt DeVos Simon Fraser	On nowhere-zero flows
26 May	Luis Goddyn Simon Fraser	Odd thrackles on surfaces
02 June	CanaDAM 2015	CANCELLED
09 June	Robert Samal Charles U.	Unique Vector Coloring and Cores
16 June	Ron Graham conference	CANCELLED
23 June	Amanda Montejano UNAM	Cyclic orders and oriented 3-hypergraphs.
30 June	Marni Mishna Simon Fraser	Efficient uniform generation of reluctant walks
07 July	Moshe Rosenfeld	The wondering salmon problem and (unrelated) graph designs
14 July	Christopher Duffy UVic	Colourings and simple colourings of (j,k)-mixed graphs
Thursday 23 July	Bruce Reed McGill University	Connectivity preserving iterative compaction
28 July	César Hernández-Cruz UNAM	Mixed cages Acyclic disconnection and minimum feedback arc sets of graphs
04 August	Jan Manuch UBC

10 May	Natalia García-Colín U Nacional Autónoma de México	On the tolerated Tverberg number
17 May	Jonathan Jedwab Simon Fraser University	How many mutually unbiased bases can exist in complex space of dimension d?
24 May	Dmitry Doryn IBS Center for Geometry and Physics	c₂ invariants of Feynman graphs
31 May	—	CANCELLED
7 Jun	—	CANCELLED
14 Jun	César Hernández Cruz U Autónoma de Zacatecas	A dichotomy for the kernel by H-walks problem in digraphs
21 Jun	Flavia Bonomo U Buenos Aires	Three-coloring and list three-coloring graphs without induced paths on seven vertices
28 Jun (extra) 11:30am	Ross McConnell Colorado State University	Algorithms on graphs arising from intersections of intervals
28 Jun	Melissa Huggan Dalhousie University	Conjoined games: go-cut and sno-go
5 Jul (extra) 11:30am	Mathew Francis Indian Statistical Institute, Chennai Centre	Uniquely restricted matchings in interval graphs
5 Jul	Hendrik Süß University of Manchester	Frobenius splitting of toric and T-varieties
12 Jul	Bruce Reed McGill University	How to determine if a random graph with a fixed degree sequence has a giant component
19 Jul	—	CANCELLED
26 Jul	Iain Crump Simon Fraser University	The extended graph permanent as an affine variety
Thursday 28 Jul 3:30pm	Mordecai Golin Hong Kong UST	Optimal binary comparison search trees

22 Sept	Ilya Shmulevich Institute for Systems Biology	Probabilistic Boolean Networks as Models of Genetic Regulatory Networks
Thursday 24 Sept 11:00am	Harrison Chapman University of Georgia	Asymptotic laws for knot diagrams
29 Sept	Gary Greaves Tohoku University	Equiangular lines in Euclidean spaces
6 Oct	Karen Yeats Simon Fraser	A few c_2 invariants of circulant graphs
13 Oct	Abbas Mehrabian UBC/SFU	Cops and a Fast Robber on Planar and Random Graphs
20 Oct	Matt DeVos Simon Fraser	Flows with large support
27 Oct	Alejandro Erickson Durham University	Graphs at the heart of the cloud
3 Nov	Eric Fusy CNRS / École Polytechnique	Baxter permutations and meanders
10 Nov	Yota Otachi Japan Advanced Institute of Science and Technology	Induced minor-free graphs: isomorphism and clique-width
17 Nov	Theodore Kolokolnikov Dalhousie University	Maximizing algebraic connectivity for certain families of graphs
24 Nov	Shenwei Huang Simon Fraser University	Colouring graphs without an induced path: A survey
1 Dec	Luis Goddyn Simon Fraser	Extending Hawiger’s Conjecture and Bicircular Matroids
8 Dec	Campus closure	CANCELLED
Monday 14 Dec PIMS 8500 1:30pm	Chính Hoàng Wilfred Laurier University	On the structure of (pan, even hole)-free graphs
Monday 14 Dec PIMS 8500 3:00pm	Mark Giesbrecht U Waterloo	Eigenvalues, invariant factors and random integer matrices (with a little bit of computing)

Tuesday Jan 13	David Gajser, University of Ljubljana, Slovenia	Simple PTAS’s for families of graphs excluding a minor
Tuesday Jan 20	Martina Mockovciakova (University of West Bohemia, Pilsen, Czech Republic)	Star Edge Coloring of Some Classes of Graphs
Tuesday Jan 27 (IRMACS, 12:30 reception)	Michael Singer (NCSU) (PIMS Distinguished Visitor)	Galois theory of difference and differential equations.
Tuesday Feb 3	Manuel Kauers (RISC)	Walks in the Quarter Plane with Multiple Steps
Tuesday Feb 10	(Reading Week)
Tuesday Feb 17	Cedric Chauve, SFU	Graph-theoretical algorithms to correct gene trees
Tuesday Feb 24	Edita Rollova (KMA)	Homomorphisms and colourings of signed graphs
Tuesday Mar 3	Murilo da Silva	Even-hole-free graphs
Tuesday Mar 10	Henry Liu, New University Lisbon, Portugal	Connected subgraphs in edge-coloured graphs
Tuesday Mar 17	Ashok Rajaraman	Vertex Ordering Problems for Hypergraphs: Connections to the Consecutive Ones Property
Tuesday Mar 24	Veronika Irvine (U Victoria)	Lace patterns
Tuesday Mar 31	Guillaume Chapuy (Liafa/ Paris 7)	Spanning trees of the graph of spanning trees.
Tuesday April 7	Karen Yeats (SFU)	Rooted tree classes and the renormalization group equation
Tuesday April 14	Nathan Ilten (SFU)	Counting lines on toric surfaces

Discrete Math Seminars

2024 Talks

Spring 2024

Fall 2023

spring 2023

Fall 2022

Spring 2022

Fall 2021

Summer 2021

19 May

Joy Morris, University of Lethbridge

2 June

Xiang-dong Hou, University of South Florida

8 June

*Tuesday @ 2 pm*

Special session for the Round the World Relay in Combinatorics

Fan Chung, University of California San Diego

16 June

Akbar Rafiey, SFU

23 June

Rebecca Patrias, University of St Thomas

30 June

Haggai Liu

7 July

Christin Bibby, Louisiana State University

14 July

Yifan Jing, UIUC

21 July

Sophie Spirkl, Waterloo

28 July

Anurag Bishnoi, TU Delft

Spring 2021

20 January

Peter Bradshaw, SFU

27 January *10am Start*

Ambat Vijayakumar, Cochin University of Science and Technology, India

3 February

Damanvir Binner, SFU

10 February

Pengyu Liu, SFU

17 February Reading Break

24 February

Harmony Zhan, York University

2 March

*TuesDAY @ 11:30-13:30*

SpeCial Session: Nancy Ann Neudauer, Pacific University

10 March

Zeying Wang, Michigan Tech

17 March

Riana Roux, Stellenbosch University

24 March

Daniel Panario, Carleton University

31 March

Tomáš Masařík, SFU

7 April

Amy Wiebe, Free University of Berlin

14 April

Carolyn Chun, United States Navel Academy

Fall 2020

Spring 2020

Fall 2019

Spring 2019

Fall 2018

Summer 2018

Spring 2018

Fall 2017

Summer 2017

Spring 2017

Fall 2016

Summer 2016

Spring 2016

Fall 2015

Summer 2015

Spring 2015

Seminars 2001-2010

Contact us

Tuesday @ 2 pm

27 January
10am Start

TuesDAY @ 11:30-13:30